1. Field of the Invention
The invention relates to a device and a method for wavefront measurement of an optical imaging system, and to a microlithography projection exposure machine having a projection lens and comprising such a device.
2. Description of the Related Art
Methods and devices for wavefront measurement are used variously for determining aberrations of optical imaging systems, in particular ultra-precision projection lenses in microlithography projection exposure machines. During wavefront measurement, deviation of the surface shape is determined with reference to an ideal surface shape. Such a deviation is denoted as a wavefront abberation. The imaging quality of an imaging sys-tem can be characterized by determining such aberrations, which can be described, for example, at all field points with the aid of Zernike coefficients. A set of Zernike coefficients can then be determined for each field point, and thus a field distribution can be specified for each Zernike coefficient. This enables a complete description of the spatially low-frequency behavior of all the aberrations of an imaging system.
Laid-open specification DE 101 09 929 A1 describes a device for wavefront measurement of an optical system that has a wavefront source with a two-dimensional structure for generating a wavefront that traverses the optical system, a diffraction grating behind the optical system, and a spatially resolving detector arranged behind the diffraction grating. A shearing interferometry technique in which the diffraction grating is laterally displaced is used for the wave-front measurement.
In addition to the type of wavefront measurement devices outlined above, which operate according to the principle of shearing interferometry, use is frequently made of a second type, which is based on the principle of the Shack-Hartmann pupil shearing. U.S. Pat. No. 5,978,085 describes such a method based on the Shack-Hartmann principle for analyzing an imaging lens system by measuring wavefront aberrations. In this method, a reticle having a structure composed of several small openings is introduced into the object plane. An aperture stop with at least one opening is positioned at a suitable spacing from said reticle. The reticle is imaged through the aperture stop onto the image plane of the lens system, in which a plurality of light spots are produced. In one embodiment the structure of the light spots is recorded by means of a wafer coated with photoresist. The displacements of the measured centroid positions as compared with the ideal, diffraction-limited ones of the light spots are determined by comparing the structures on the wafer with a reference plate that is brought to overlap the wafer and has been exposed with reference structures. These displacements are used to determine the gradient of the wavefront in the pupil of the lens system to be measured, and therefore the aberration of the wave-front.
U.S. Pat. No. 5,828,455 describes a similar method based on the Shack-Hartmann principle. In one embodiment of this method, the structure of the light spots is recorded on a chromium-coated fused silica wafer. The displacements of the structures with reference to the respective ideal position are measured with the aid of an optical measuring instrument. An exposure process for the wafer is required in the case of this method, as well.
One object of the invention is to provide a device and a method that render possible a wavefront measurement of an optical imaging system with relatively low outlay. Moreover, the invention is based on creating a microlithography projection exposure machine that comprises such a device.